1. Field of the Invention
The present invention relates to an optical circuit of a variable optical gain control device, which is used for controlling the luminance intensity in an optical communication system, and also to the structure of the optical circuit.
2. Description of the Related Art
Nowadays, a lot of the optical communication systems in the world introduce a Wavelength Division Multiplexing (WDM) system as the most effective systems for increasing the optical transmission capacity. Heretofore, in the WDM system, as a means for arbitrarily controlling the luminance intensity to an arbitrary value, there has been used a Variable Optical Attenuator (VOA) having the function of changing the amount of attenuation of the luminance intensity. In view of the increased tendency of such an optical communication system to be provided with the WDM system, there is being still vigorously carried out the technological development on such a VOA that is able to change the attenuation of the luminance intensity in response to an electrical signal.
In the WDM system, the VOA is used for two principal purposes. One is to equalize the level of the optical signal which is different on every wavelength thereof, and the other is to keep the output level of the optical signal constant even if the wavelength and the input level of the optical signal to be amplified are changed when the optical amplification factor is made constant in a multi-wavelength collective optical amplifier. The reason for keeping the optical amplification factor constant is for preventing the deterioration of the system characteristic caused by variation in the wavelength characteristic of the optical amplifier
As one prior art VOA, there is a VOA of the non-mechanical type which makes use of the thermo-optic effect. Typically, the VOA of this type is provided with a Mach-Zehnder interferometer formed on a glass-made optical waveguide path. Each of two optical paths of this interferometer is provided with a heater, and the phase difference of the optical signal in each waveguide path between these optical paths is changed by the temperature difference between two heaters. The VOA of this type can be arranged in the form of an array with ease, and the VOA having 20 to 40 channels is now being produced and already on the market.
In case of executing the optical gain control by means of the prior art VOA, however, as the luminance intensity is controlled by attenuating it, there occurs such an inconvenience that the luminance intensity can not help being smaller than the original one.
On the one hand, if it is tried to control the optical gain by mean of an Erbium Doped Fiber Amplifier (EDFA), problems to be considered would come up. For instance, large noise generation, or an increase in the dimension of the entire system may increase because the optical fiber has to be wound many times before putting it in a predetermined package case, and so forth. Furthermore, in case of controlling the optical gain by mean of a semiconductor optical amplifier, the coupling loss between the amplifier and the optical fiber becomes so large that dependency on a biased wave is made stronger.